50HZ AC track circuits

[PJW]

Hello again, I have been reading up on the above subject, and was wondering why as a general rule of thumb that trains hit the relay end of a TC first?

For any track circuit there is a difference between the "drop shunt" and "pick shunt"; i.e. the train must present a better short circuit to drop a track initially than to keep it down. The most effective way of shorting out the current that would otherwise go through the TR is by applying the parallel resistance as close to its coils as possible.

ac vane track circuits tend to be used on dc electrified railway and there inevitably has to be a means of getting traction current back to the substation. This has to flow through the running rails and will of course create a voltdrop as it does so since rail doed have some resistance and indeed impedance (dc traction isn't perfectly smooth dc). If the current down one rail isn't 100% balance by that in the other rail, then there will be differential voltage as a result and it only needs a small voltage to pick a track relay. That's why we ise ac vanes to give immunity to d, but of course there will always be an ac component in the traction supply.

In order to attempt to separate the ac from the dc, then we use devices called impedance bonds which try to block the ac track circuit voltage whilst permitting as low resistance path to the dc traction so that it can flow back in the two (or indeed also those of adacent tracks) running rails. Inevitably they are not pefect and the effect is the same as low ballast resistance; hence can't achieve a high drop shunt and yes we do have to accept 0.3ohm rather than the normal absolute minimum of 0.5 ohm and the preferred 0.9 ohm.

Therefore it is particularly important for these tracks that the train presents to the relay end first; it is a risk mitigation measure to arrange things that the predominant traffic flow does so. It is worth remembering two other mitigations- such railway is generally well used and rarely has problems with rusty rails (though mulch in leaf fall season is something else!), also that most of the trains are relatively long with a corresponding number of axles. Further the fact that the impednace bond create a low ballast resistance all the time means that the effect on te track circuit by variations (due to weather) of the real ballast resistance is much less significant and therefore the same margins of safety are not required to take this variation into account.

Good question; well done for recognising the issue and raising the query. Hope this response answers it adequately.